1. Field of the Invention
The present invention relates to a power supply loss sensor.
2. Description of the Prior Art
Battery-operated electronic devices are becoming increasingly sophisticated, with microprocessors and other types of integrated logic circuitry being used in a variety of portable, mobile, and other applications. Often a primary battery is used in normal operation, with a backup battery retaining essential data and circuit functions when needed; for example, when the primary battery becomes too weak to function, or when the primary battery is being replaced. Referring to FIG. 8, data corruption or system malfunction due to loss of the primary battery (80) is commonly prevented by using a "diode-switch" method. This method uses two diodes (81,82) external to the system device (e.g., integrated circuit 83), and connected so that one diode is reverse biased while the other diode is forward biased. This type of solution is acceptable if the voltage level of V.sub.DD is high enough so that a diode voltage drop from V.sub.DD will still provide a solid power supply voltage to the system, which is usually true if V.sub.DD is 5 V (plus or minus 10 percent). However, in the case of a 3 V (plus or minus 5 percent) V.sub.DD, using this method, the power supply voltage to the integrated circuit 83 is in the range of only 2.2 V to 2.5 V (i.e., V.sub.DD minus the diode voltage drop). This level of power supply voltage (2.2 V to 2.5 V) may degrade the performance of a logic circuit to a point which is not acceptable. For example, the speed degradation due to the lowering of the power supply from 3 V to 2.2 V is typically a factor of 2.
One method of reducing the voltage loss across a diode is to switch between the batteries with transistors. For example, U.S. Pat. No. 4,451,742 shows one such circuit. However, in the voltage loss detector discloses therein, analog circuitry is used to compare the primary power supply to one (or more) backup batteries. Hence, an analog process technology is required when implementing the circuit in integrated circuit form. Furthermore, the disclosed circuitry may also result in significant current drain on the power supply. In another example, the use of an analog comparator in a battery switch-over circuit is shown in the Low Voltage Databook of National Semiconductor, 1992 Edition, at page 3-57.